An updated and refined catalog of earthquakes in Taiwan (1900–2014) with homogenized M w magnitudes
© Chang et al. 2016
Received: 22 October 2015
Accepted: 13 February 2016
Published: 17 March 2016
The Erratum to this article has been published in Earth, Planets and Space 2016 68:122
The main goal of this study was to develop an updated and refined catalog of earthquakes in Taiwan (1900–2014) with homogenized M w magnitudes that are compatible with the Harvard M w . We hope that such a catalog of earthquakes will provide a fundamental database for definitive studies of the distribution of earthquakes in Taiwan as a function of space, time, and magnitude, as well as for realistic assessments of seismic hazards in Taiwan. In this study, for completeness and consistency, we start with a previously published catalog of earthquakes from 1900 to 2006 with homogenized M w magnitudes. We update the earthquake data through 2014 and supplement the database with 188 additional events for the time period of 1900–1935 that were found in the literature. The additional data resulted in a lower magnitude from M w 5.5–5.0. The broadband-based Harvard M w , United States Geological Survey (USGS) M, and Broadband Array in Taiwan for Seismology (BATS) M w are preferred in this study. Accordingly, we use empirical relationships with the Harvard M w to transform our old converted M w values to new converted M w values and to transform the original BATS M w values to converted BATS M w values. For individual events, the adopted M w is chosen in the following order: Harvard M w > USGS M > converted BATS M w > new converted M w . Finally, we discover that use of the adopted M w removes a data gap at magnitudes greater than or equal to 5.0 in the original catalog during 1985–1991. The new catalog is now complete for M w ≥ 5.0 and significantly improves the quality of data for definitive study of seismicity patterns, as well as for realistic assessment of seismic hazards in Taiwan.
Taiwan is located at the juncture of the Philippine Sea plate on the east side and the Eurasian plate on the west side. The Longitudinal Valley in eastern Taiwan marks the suture zone of the two colliding plates. The Philippine Sea plate subducts northwardly under the Eurasian plate along the Ryukyu Trench in the eastern offshore region of Taiwan. Along the Manila Trench in the southern offshore region of Taiwan, the South China Sea Basin on the Eurasian plate subducts eastwardly under the Philippine Sea plate. As a result, earthquakes occur very frequently in this region, many of which are disastrous. Therefore, it is necessary to compile an earthquake catalog with consistent magnitudes to facilitate meaningful delineation of seismicity patterns and reliable assessment of seismic hazards in Taiwan.
A complete catalog of earthquakes with consistent magnitudes can provide necessary data for studying the distribution of earthquakes in a region as a function of space, time, and magnitude. Unfortunately, most catalogs do not determine the magnitude of earthquakes consistently over time. This inconsistency may compromise the usefulness of earthquake catalogs.
Taiwan has a relatively complete catalog of earthquakes since the first seismograph was installed in 1897. However, due to changes in seismographic characteristics, network coverage, and observational practice, the definitions and procedures for determining the magnitude have changed over time. Chronologically, M H was determined using intensity data for earthquakes in 1900–1972. M D(A) was determined using the total signal duration on analog seismograms for earthquakes in 1973–June 1987. M D(D) was determined using the total signal duration on digital seismograms for earthquakes in June 1987–February 1991. The local magnitude M L has been determined using simulated Wood-Anderson seismograms for earthquakes from March 1991–present. Hence, in order to provide good data for meaningful delineation of seismicity patterns and reliable assessment of seismic hazards in Taiwan, we need to convert a raw catalog of earthquakes with magnitudes determined on mixed scales to a modified catalog of earthquakes with internally consistent magnitudes. Toward this end, we develop a catalog of Taiwan earthquakes with homogenized M w magnitudes covering the time period of 1900–2006 (Chen and Tsai 2008).
Since our previous version of the Taiwan earthquake catalog was published, several developments have motivated us to further update and refine it in this study. We first want to add earthquake data acquired in the last 8 years from 2007 to 2014. Equally significantly, we found in an unpublished report by Hsu (1989) an additional 188 events during the early time period of 1900–1935 that were not included in the previous catalog. The addition of these missing events resulted in a reduction of the lower cutoff magnitude threshold from M w 5.5 to M w 5.0 for the time period 1900–1935.
In terms of refining the catalog, we strive to adjust the magnitude to be more compatible with the Harvard M w using the process described below. In the end, we hope that the newly updated and refined catalog of Taiwan earthquakes (1900–2014) provides a superior database for quantitative seismicity studies.
Conversion from original magnitudes to old M w magnitudes
1900–1972 dataset: As mentioned above, M H magnitude was determined based on intensity data for this subset of Taiwan earthquakes. For magnitude conversion, we first determined a relationship between M H and M S based on common earthquakes in both the Taiwan catalog and the National Earthquake Information Center (NEIC) catalog. We next found a relationship between M S and M w based on common earthquakes in the NEIC catalog. We then combined these two relationships to obtain a relationship between M w and M H , as follows:
1973–1987.6 dataset: As mentioned above, the M D(A) magnitude was determined from the total signal duration measured on analog seismograms for this subset of Taiwan earthquakes. For magnitude conversion, we found a relationship between M D(A) and M w based on common earthquakes in this time period listed in both the NEIC and Taiwan catalogs, as follows:
1987.6–1991.2 dataset: As mentioned above, the M D(D) magnitude was determined from the total signal duration measured on digital seismograms for this subset of Taiwan earthquakes. For magnitude conversion, we first found a relationship between M D(D) and M w based on common earthquakes listed in both the NEIC and Taiwan catalogs. Since the number of M D(D) and M w values was too small to compare identical events, we adopted the same slope as the relationship between M D(A) and M w . This resulted in the following conversion formula:
1991.3–2006 dataset: As mentioned above, the M L magnitude was determined from simulated Wood-Anderson seismograms for this subset of Taiwan earthquakes. For magnitude conversion, we first found a relationship between M L and M w based on common earthquakes listed in both the NEIC and Taiwan catalogs, as follows:
This relationship is also shown in Fig. 1. From the figure, we can see that the M L magnitude is slightly larger than M w . However, the M L magnitude scale tends to saturate beyond M L ≥ 6.5, as shown by solid dots in Fig. 1. Finally, Eq. 4 was used to convert M L to M w for earthquakes from 1991.3 to 2006.
Choice of adopted M w magnitudes for Taiwan earthquakes from 1900 to 2014
This study aims to update and further refine our previous catalog of Taiwan earthquakes (1900–2006) with homogenized M w magnitudes that were converted from four different types of original magnitude (Chen and Tsai 2008). The original magnitude scales for Taiwan earthquakes from 1900 to present were based on either intensity data or short-period seismograms. As a result, their conversion to the broadband-based moment magnitude M w will inevitably encounter the saturation problem for large earthquakes. In order to circumvent this obstacle, we use a priority list to adopt a preferred M w for individual earthquakes in the newly updated catalog from 1900 to 2014.
Another moment magnitude, BATS M w , is also routinely determined using the records of the Broadband Array in Taiwan for Seismology (BATS) operated by the Institute of Earth Sciences of Academia Sinica since July 1995. Although the BATS M w is determined by a similar procedure as the Harvard M w , the extracted surface wave period is shorter than that used by the Harvard M w . This results in a systemic difference between BATS M w and Harvard M w , as shown in Fig. 3b. The empirical relationship in the figure is used to convert BATS M w to Harvard M w , denoted as “converted BATS M w .” In this study, we also use the United States Geological Survey (USGS) catalog and find that the USGS M is almost the same as the Harvard M w , as shown in Fig. 3c. Thus, the original USGS M is retained.
The Harvard M w is adopted whenever available.
Available USGS M is adopted when the Harvard M w is not available.
Available converted BATS M w is adopted when neither Harvard M w nor USGS M is available.
New converted M w is adopted when none of the above three M w is available.
The preferred M w chosen according to the list above is denoted as the “adopted M w .” Additional file 1: Table S1 lists a total of 1989 earthquakes with adopted M w ≥ 5.0 that occurred in the Taiwan region from 1900 to 2014.
Merits of the 188 supplementary events and new adopted M w
Distribution of earthquakes in Taiwan
In order to develop a more complete catalog of Taiwan earthquakes with magnitude M w ≥ 5.0, we have merged the four subcatalogs with mixed magnitude scales. Taking the Harvard M w as our final goal, we have chosen the adopted M w for individual earthquakes according to the following order: Harvard M w > USGS M > converted BATS M w > our new converted M w .
List of earthquakes with a magnitude ≥7.0 for comparison among our catalog M w , Harvard M w , and the United States Geological Survey (USGS) catalog during 1973–2014
New converted (M w )
Harvard (M w )
Conclusion and discussion
A complete catalog of earthquakes with homogenized moment magnitude M w can provide indispensable data for studying the distribution of earthquakes in a region as a function of space, time, and magnitude. With this in mind, the present study aims to convert mixed original magnitude values of Taiwan earthquakes from 1900 to 2014 to a homogenized M w that is closely compatible with the Harvard M w . Before 1973, graphical methods were used to locate earthquakes in Taiwan using smoked paper records with manual timing from pendulum clocks at individual seismic stations. In order to recover missing earthquake data from this early period, we conducted a literature search. Fortuitously, we found data on 188 additional earthquakes from 1900 to 1935 in an unpublished report by Hsu (1989). These supplementary events have enriched the catalog with earthquakes with a magnitude ≥5.0 during the time period 1900–1935. New events from 2007 to 2014 were also added to bring the catalog of earthquakes in Taiwan up to date.
Recognizing that our new converted M w values from previous study are based on old intensity data or recent short-period seismograms, either the broadband-based Harvard M w , USGS M, or converted BATS M w is chosen as the adopted M w for individual events in the following order: Harvard M w > USGS M > converted BATS M w > our new converted M w .
After converting the original magnitudes of earthquakes in Taiwan to new converted M w values, the gap in the data for earthquakes with magnitudes ≥5.0 from 1985 to 1991 is eliminated. In the meantime, the new converted M w values approach the Harvard M w values. We also compare our new converted M w with USGS M and Harvard M w for identical events with magnitudes ≥7.0 and find that our new converted M w values are often smaller than the Harvard M w or USGS M values, with the largest difference reaching 1.0. This is primarily due to saturation of the original magnitude scales above certain magnitudes. Although we try to convert the original magnitudes to new converted M w values, this shortcoming is still not overcome.
W-YC dealt with data processing, K-PC provided the text and illustrations of the manuscript, and Y-BT set the key themes of the manuscript. All authors read and approved the final manuscript.
The authors wish to express their sincere appreciation to the Central Weather Bureau and Institute of Earth Sciences for providing the original catalog of earthquakes in Taiwan. This study was supported by Grant NSC101-2116-M-259-006-MY4.
The authors declare that they have no competing interests.
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